The key point here is :
We know a tree is a minimally connected graph..So if one less the number of edges in a tree is there , then it is always guaranteed that the resultant graph will be disconnected..
So given m vertices , n edges..
Hence no of edges in tree = m - 1
Hence one less will guarantee that the graph will be disconnected i.e. = m - 2..
Now initially we have n edges..Let no of edges to be removed = x..Hence
n - x = m - 2
==> x = n - m + 2
Hence D) is correct answer..